The present invention relates to a broken-wire failure detection circuit with a failure detection function for outputting a voltage out of a normal output-voltage range as a dialog voltage when a broken-wire failure occurs in a power line, signal line, or ground line of an electric or electronic circuit connected to a higher circuit.
FIG. 16 is a circuit diagram showing a configuration of a conventional broken-wire failure detection circuit. As shown in FIG. 16, the conventional broken-wire failure detection circuit includes a functional circuit 1 composed of an electric or electronic circuit in which the first resistance means 21 is connected between a power line 11 and a signal line 12, and the second resistance means 22 is connected between the signal line 12 and a ground line 13.
The power line 11 of the functional circuit 1 is connected to a power line 41 of a higher circuit 4 that supplies power to the functional circuit 1. The signal line 12 of the functional circuit 1 is connected to a signal output terminal 45 via a signal line 42 of the higher circuit 4. A signal output-circuit section 3 is provided in the functional circuit 1. A signal from a signal output-circuit output stage amplifier 31 in the signal output-circuit section 3 is output from a signal output terminal 45 via the signal lines 12 and 42. The ground line 13 of the functional circuit 1 is connected to a ground line 43 of the higher circuit 4.
In the higher circuit 4, an internal resistance between the power line 41 and the signal line 42 constitutes the fourth resistance means 24. Further, an internal resistance between the signal line 42 and the ground line 43 constitutes the fifth resistance means 25. The power line 41 and ground line 43 of the higher, circuit 4 are connected to a power terminal 44 and a ground terminal 46.
When the broken-wire failure detection circuit configured as shown in FIG. 16 is applied to, for example, a semiconductor pressure sensor of a vehicle, a power voltage Vcc of 5 V is supplied to a sensor constituting the functional circuit 1. FIG. 17 schematically shows the output-voltage characteristics of the semiconductor pressure sensor. As shown in FIG. 17, if there is no broken-wire failure, the output voltage Vout from the sensor varies within a normal output-voltage range 7 between, for example, 0.5 V and 4.5 V, depending on the applied voltage.
If a broken-wire failure occurs in the power line, signal line, or ground line of an interface section connected to an ECU (Electronic Control Unit) constituting the higher circuit 4, the sensor output voltage Vout shifts to a voltage range 5 (hereinafter referred to as the “lower dialog-voltage range ”) lower than the normal output-voltage range 7, or a voltage range 6 (hereinafter referred to as the “upper dialog-voltage range ”) higher than the normal output-voltage range 7.
For the convenience of the description, the resistance values of the first, second, fourth, and fifth resistance means 21, 22, 24, and 25 are defined as R1, R2, R4, and R5, respectively. A composite resistance value obtained by adding internal resistance values R01, R02, and R03, or a composite resistance value when resistance means with resistance values Ra and Rb, respectively, are connected together in parallel, is defined as Ra//Rb. A composite resistance value is defined as Ra//Rb//Rc when resistance means with resistance values Ra, Rb, and Rc are connected together in parallel.
When a connection between the power line 11 of the functional circuit 1 and the power line 41 of the higher circuit 4 is broken, the output voltage Vout is determined from a partial voltage ratio of the resistance value R4 of the fourth resistance means 24 to the composite resistance value of the parallel connection between the second resistance means 22 and the fifth resistance means 25 (R2//R5). Accordingly, the voltage Vout is expressed by Equation (1) shown below.Vout={R2//R02//R5//(R1//R01+R03)}/{R2//R02//R5//(R1//R01+R03)+R4}×Vcc  (1)
In this case, a value of the voltage Vout is within the lower dialog-voltage range 5 and thus smaller than 0.5 V. Accordingly, the second resistance means 22, constituting a broken-wire failure detection circuit, has its resistance value R2 selected so as to satisfy Equation (2) shown below.{R2//R02//R5//(R1//R01+R03)}/{R2//R02//R5//(R1//R01+R03)+R4}×Vcc<0.5  (2)
On the other hand, when a connection between the ground line 13 of the functional circuit 1 and the ground line 43 of the higher circuit 4 is broken, the output voltage Vout is determined from a partial voltage ratio of the resistance value R5 of the fifth resistance means 25 to the composite resistance value (R1//R4) of the parallel connection between the first resistance means 21 and the fourth resistance means 24. Accordingly, the voltage Vout is expressed by Equation (3) shown below.Vout=R5/{R1//R01//R4//(R2//R02+R03)+R5}×Vcc  (3)
In this case, a value of the voltage Vout is within the upper dialog-voltage range 6 and thus higher than 4.5 V. Accordingly, the first resistance means 21, constituting a broken-wire failure detection circuit, has its resistance value R1 selected so as to satisfy Equation (4) shown below.R5/{(R1//R01//R4)//(R2//R02+R03)+R5}×Vcc>4.5  (4)
Recently, as an electronic control in automobiles has advanced, the normal output-voltage range 7 of the sensor has expanded to, for example, between 0.3 V and 4.8 V. Correspondingly, the lower dialog-voltage range 5 must be below the value of 0.3 V. To this end, the current resistance value R2 of the second resistance means 22 must be reduced. Likewise, the upper dialog-voltage range 6 must be above 4.8 V. Thus, the current resistance value R1 of the first resistance means 21 must be reduced.
When the resistance value R2 of the second resistance means 22 is reduced, it is necessary to increase a source current to the signal output-circuit output stage 31 of the functional circuit 1 during a normal operation. A transistor size or the like must be increased, thereby increasing a scale of the circuit. The scale of the circuit is also increased when the resistance value R1 of the first resistance means 21 is reduced. It is necessary to increase a sink current from the signal output-circuit output-stage amplifier 31 of the functional circuit 1 during a normal operation as well. Therefore, a transistor size or the like must be increased, thereby increasing the scale of the circuit.
The present invention is provided in view of these problems. It is an object of the present invention to provide a broken-wire failure detection circuit that can reduce the upper-limit value of the lower dialog-voltage range, and increase the lower-limit value of the upper dialog-voltage range without restrictions on a load-driving ability of the sink or source current in the signal output circuit.
Further objects and advantages of the invention will be apparent from the following description of the invention.